1. Field of the Invention
The present invention relates to a recording apparatus for image formation on plural recording members with laser beams.
2. Description of the Prior Art
In conventional electrophotographic recording apparatus such as laser beam printers, image formation is achieved by scanning an electrophotographic photosensitive member constituting a recording member with a laser beam which is modulated with image signal and is deflected by a rotary prism or a mirror (hereinafter collectively called scanner). Such photosensitive member in the conventional apparatus is usually drum-shaped.
In order to obtain a color printer with the above-mentioned process, it becomes necessary to superpose plural images of different colors on a recording sheet, according to one of the following principal arrangements.
In one arrangement, a latent image formed by scanning the photosensitive drum with the image signal of a first color is rendered visible with developer, and the obtained image is transferred onto a recording sheet. Thereafter the photosensitive drum is cleaned, and a latent image is formed on the same drum according to the image signal of a second color and is processed in the same manner, but with developer of a second color. Thereafter the same process is repeated for the image signals of a third color and a fourth color. In this manner an image is obtained by superposing plural developed images on the same recording sheet.
In another arrangement, there are provided plural photosensitive drums of a number corresponding to the number of image signals. Latent images are formed on said drums respectively corresponding to the image signals of different colors, then developed respectively with developers of different colors and transferred in succession onto the recording sheet. In such arrangement there are usually provided a laser, a scanner and a photosensitive drum for each image signal, so that plural image signals to be superposed require a corresponding number of lasers, scanners and photosensitive drums.
The first-mentioned arrangement is associated with a drawback of a very long printing time required for a printout because the electrophotographic process consisting of the steps of charging, exposure, development, image transfer and cleaning has to be carried out at first for the image signal of the first color, then to be repeated for the images signals of the second, third and fourth colors. Though being featured by a shorter printing time, the second-mentioned arrangement is again associated with a drawback of requiring a larger and more expensive apparatus, since the lasers, scanners and drums are required in a number corresponding to the number of color image signals.
Also the printing time of a laser beam printer can be shortened by increasing the revolving speed of the scanner. In the conventional laser beam printers, however, the revolving speed of the scanner is already in a range as high as 2,000 to 20,000 rpm. Besides such scanner is provided with a prism or a mirror of polygonal shape, usually octagonal shape, of which face angles have to be extremely precise, generally within a tolerance of several seconds, since even a small error in the deflection angle leads to a significant positional error on the photosensitive drum because of a relatively long optical path of the laser beam. In addition such prism or mirror has to be free of vibration as far as possible at the high-speed rotation. For such stable high-speed rotation of the polygonal mirror there is required a large motor, and the manufacture of the scanner with highly precise mirror face angles requires the use of precision working with a limited production yield. For these reasons such scanner has inevitably been very expensive.
Consequently an apparatus employing plural units of such scanners has been inevitably large and expensive.